[thefz]

TBH if eating junk food harms a rat, that can live off literal trash, imagine what it does to a human whose dietary requirements are more refined.

Makes sense empirically, malnutrition is known to have a large impact on later life, I can imagine the same is true for bad nutrition as well.

[Terr_]

I would challenge the assumption that rats have evolved notably-superior stomachs or immune systems or whatever.

Just because the species can survive scavenging human trash doesn't mean individuals aren't running risks and getting hurt. Just because I always see some rats around the dumpster doesn't mean everything's going well for them.

[thefz]

I would challenge your challenge mainly because I don't think that rats have superior immune systems because of eating whatever, but rather they eat whatever due to their superior immune system.

"We assume that many wild animals are resource limited, and so expect that immune responses may be submaximal. However, in truth, high levels of immune responsiveness are, in general, seen in wild rodents." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5694458/

It's part of why they are successful scavengers.